The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typical to subject it to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the subsequently applied radiation-sensitive coating and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining affects both the performance and the durability of the printing plate, and the quality of the graining is a critical factor determining the overall quality of the printing plate. A fine, uniform grain that is free of pits is essential to provide the highest quality performance.
Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Optimum results are usually achieved through the use of electrolytic graining, which is also referred to in the art as electrochemical graining or electrochemical roughening, and there have been a great many different processes of electrolytic graining proposed for use in lithographic printing plate manufacturing. Processes of electrolytic graining are described in numerous references.
In the manufacture of lithographic printing plates, the graining process is typically followed by an anodizing process, utilizing an acid such as sulfuric or phosphoric acid, and the anodizing process is typically followed by a process that renders the surface hydrophilic such as a process of thermal silication or electrosilication. The anodization step serves to provide an anodic oxide layer and is preferably controlled to create a layer of at least 0.3 g/m.sup.2. Processes for anodizing aluminum to form an anodic oxide coating and then hydrophilizing the anodized surface by techniques such as silication are very well known in the art, and need not be further described herein.
Illustrative of the many materials useful in forming hydrophilic barrier layers are polyvinyl phosphonic acid, polyacrylic acid, polyacrylamide, silicates, zirconates and titanates.
The result of subjecting aluminum to an anodization process is to form an oxide layer that is porous. Pore size can vary widely, depending on the conditions used in the anodization process, but is typically in the range of from about 0.1 to about 10 .mu.m. The use of a hydrophilic barrier layer is optional but preferred. Whether or not a barrier layer is employed, the aluminum support is characterized by having a porous wear-resistant hydrophilic surface that specifically adapts it for use in lithographic printing, particularly in situations where long press runs are required.
A wide variety of radiation-sensitive materials suitable for forming images for use in the lithographic printing process are known. Any radiation-sensitive layer is suitable which, after exposure and any necessary developing and/or fixing, provides an area in imagewise distribution that can be used for printing.
Useful negative-working compositions include those containing diazo resins, photocrosslinkable polymers and photopolymerizable compositions. Useful positive-working compositions include aromatic diazooxide compounds such as benzoquinone diazides and naphthoquinone diazides.
Lithographic printing plates of the type described hereinabove are usually developed with a developing solution after being imagewise exposed. The developing solution, which is used to remove the non-image areas of the imaging layer and thereby reveal the underlying porous hydrophilic support, is typically an aqueous alkaline solution and frequently includes a substantial amount of organic solvent. The need to use and dispose of substantial quantities of alkaline developing solution has long been a matter of considerable concern in the printing art.
Efforts have been made for many years to manufacture a printing plate that does not require development with an alkaline developing solution. Examples of the many references relating to such prior efforts include, among others: U.S. Pat. No. 3,506,779 (Brown et al), U.S. Pat. No. 3,549,733 (Caddell), U.S. Pat. No. 3,574,657 (Burnett), U.S. Pat. No. 3,793,033 (Mukherjee), U.S. Pat. No. 3,832,948 (Barker), U.S. Pat. No. 3,945,318 (Landsman), U.S. Pat. No. 3,962,513 (Eames), U.S. Pat. No. 3,964,389 (Peterson), U.S. Pat. No. 4,034,183 (Uhlig), U.S. Pat. No. 4,054,094 (Caddell et al), U.S. Pat. No. 4,081,572 (Pacansky), U.S. Pat. No. 4,334,006 (Kitajima et al), U.S. Pat. No. 4,693,958 (Schwartz et al), U.S. Pat. No. 4,731,317 (Fromson et al), U.S. Pat. No. 5,238,778 (Hirai et al), U.S. Pat. No. 5,353,705 (Lewis et al), U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,395,729 (Reardon et al), EP-A-0 001 068, and EP-A-0 573 091.
Lithographic printing plates designed to eliminate the need for a developing solution that have been proposed heretofore have suffered from one or more disadvantages that have limited their usefulness. For example, they have lacked a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas with the result that image quality on printing is poor, or they have had oleophilic image areas which are not sufficiently durable to permit long printing runs, or they have had hydrophilic non-image areas that are easily scratched and worn, or they have been unduly complex and costly by virtue of the need to coat multiple layers on the support.
The lithographic printing plates described hereinabove are printing plates that are employed in a process which employs both a printing ink and an aqueous fountain solution. Also well known in the lithographic printing art are so-called "waterless" printing plates that do not require the use of a fountain solution. Such plates have a lithographic printing surface comprised of oleophilic (ink-accepting) image areas and oleophobic (ink-repellent) background areas. They are typically comprised of a support, such as aluminum, a photosensitive layer that overlies the support, and an oleophobic silicone rubber layer that overlies the photosensitive layer, and are subjected to the steps of imagewise exposure followed by development to form the lithographic printing surface. Such printing plates can be directly imaged using lasers. In such instances, laser imaging typically "ablates", or partially or totally removes or loosens one or more layers in the exposed areas.
While such materials and imaging methods have considerable utility, there remains a need to remove or dispose of the "ablated" debris (that is, ablated or loosened debris from the layers) from the printing plates before inking. This can be done by wiping or washing with a solvent, or other mechanical means, as described for example in U.S. Pat. No. 5,378,580 (Leenders). This step, while essential in conventional methods, complicates the imaging and printing process, requiring an additional process step and additional equipment and/or cleaning solutions. Hence, there is a desire to avoid the need for removing debris from the imaging process.
Ceramic printing members, including printing cylinders are known. U.S. Pat. No. 5,293,817 (Nussel et al), for example, describes porous ceramic printing cylinders having a printing surface prepared from zirconium oxide, aluminum oxide, aluminum-magnesium silicate, magnesium silicate or silicon carbide.
It has also been discovered that ceramic alloys of zirconium oxide and a secondary oxide that is MgO, CaO, Y.sub.2 O.sub.3, Sc.sub.2 O.sub.3 or a rare earth oxide are highly useful printing members, as described for example, in copending U.S. Ser. No. 08/576,178 (noted above).
There are some "erasable" plates known in the art that can be reused, but they have not gained high acceptance for a number of reasons. It would be desirable to have a means for printing one or more images on the same lithographic printing member without the need for debris removal.
Moreover, while the zirconia alloy ceramics described above provide a number of advantages, there is a need to provide a means of imaging improved ceramics without the need for debris removal, which ceramics have higher strength, fracture toughness and wearability.